This invention relates to an apparatus and method for aligning an integrated circuit (IC) package in preparation for soldering it to the second level of packaging, e.g., a printed circuit board. More particularly, it relates to an apparatus and method that aligns the leads of the IC package with the pads of the second level package and maintains this alignment while the leads of the IC package are bonded to the desired pads.
Electronic products that utilize IC's typically comprise several levels of packaging. The first level of packaging is the IC package which contains the IC chip itself and has leads (also called legs or pins) that allow it to be both electrically and mechanically connected to the next level of packaging. Fine wires are used within the package to connect the signal pads of the chip to the leads of the package.
The second level of packaging provides a means of interconnecting the IC's. Traditionally, epoxy-fiberglass printed circuit boards, quite often with multiple layers of wiring, have been used for this purpose. The IC package is attached, by a variety of means, to the printed circuit board to make an electrical connection between the leads of the package and the wiring of the board and, at the same time, mechanically attach it to the second level package.
Small scale integration (SSI) and medium scale integration (MSI) IC's are typically packaged in dual-inline packages (DIP's). These packages are rectangular in shape and have a relatively small number of leads. Typical totals are 14 or 16, while a few DIPs have as many as 40 leads. The leads are usually spaced a tenth of an inch apart, with half of the total leads being placed along each long dimension of the DIP. DIP's are normally attached to the printed circuit board by inserting their leads in through-plated-holes provided on the board and soldering them in place. They can be readily removed for replacement by melting and removing the solder from the holes. Techniques for doing this are well understood in the art.
When integrated circuit technology improved to large scale integration (LSI) and very large scale integration (VLSI), the number of input/output signals of the chips increased dramatically. For example, VLSI packages with 256 leads are now being used. Since the object of LSI and VLSI is to increase the density of circuits at all levels of packaging, the leads of the LSI and VLSI IC packages are made smaller in width and placed closer together than those of the DIP's.
The typical LSI or VLSI package is square with an equal number of leads on each side. Because the leads are so small and so close together, e.g., leads 0.010 inches wide with 0.010 inch spaces between them are not uncommon, it is impractical to provide through-plated-holes of similar dimensions. Even if through-plated-holes could be fabricated with those dimensions, it would be difficult to insert all the leads into all the holes simultaneously without causing damage to the leads. Once the leads were soldered in the holes, it would be very difficult to unsolder all of them without causing damage to the board if the IC package had to be replaced.
LSI and VLSI IC packages are typically attached to the printed circuit board by lap soldering the leads to pads, arranged in the same pattern as the leads, on the surface of the board. To accommodate this, the leads of an LSI or VLSI package typically radiate outward, parallel with the top and bottom surface of the package, but positioned beneath the bottom surface.
Because of the number of leads and their smallness, it is desirable in a manufacturing process to bond them to the pads of the board using automated or semi-automated techniques. (The actual bonding process used--for example, thermo-compression bonding, laser beam bonding, lap soldering, etc.--is immaterial to this invention.) However, before this automated or semi-automated process can begin, all the leads of the package must be aligned with all of the corresponding pads of the printed circuit board.
Disadvantageously, there is an inherent problem in LSI and VLSI packages which in the prior art, has required that the alignment of the leads of the package with the pads of the printed circuit board be done manually. Once this alignment is performed and the package is held in place, the automated or semi-automated bonding can take place.
Both the pads on the printed circuit board and the leads of the IC package are formed by an etching process. The pattern to be etched is first formed on film and then transferred to the board, or metal from which the leads will be made. The pattern on the film can be made with high accuracy, e.g., to tolerances of 0.001 inch and better, using automated photo-plotters. The end result is that the pads on the printed circuit board are precisely located with respect to each other, as are the leads of the IC package.
When the leads are attached to the package, they maintain their precise location with respect to each other. Unfortunately, however, because of the assembly process, the position of the leads with respect to the corners of the package may vary, from package to package, on the order of 0.01 inch.
In the past, attempts to automatically align the leads of the package to the pads of the printed circuit board have used the package as the reference for alignment. Since the leads have a potential error in placement with respect to the package, they also have a potential error in placement with respect to the pads. Hence, the common technique used in the prior art to align the leads to the pads is to place the package on the pads so that the leads are approximately aligned with the pads. This is done by some automated process or manually by an operator. The operator, using a microscope, then moves the package to obtain the "best" fit between the pads and the leads. The operator then causes a number of leads to be bonded to their respective pads, typically one near each corner of the package, to hold the package in place during the semi-automated or automated bonding process which follows.
The process of the prior art has several disadvantages, including: (1) it takes time, and in any manufacturing process, it is desirable to reduce time, and thereby, reduce costs; (2) the operator, being human, is subject to making errors in the alignment, thereby creating the possibility of having leads touching adjacent pads and causing electrical shorts; and (3) after aligning the leads and during the bonding of several leads to hold the package in place, the package can be inadvertently moved enough to cause electrical shorts.
Thus, it is apparent from the above discussion that a need exists in the art for an apparatus and method to align the leads of a VLSI or LSI chip package to the pads of the printed circuit board to which the package is to be bonded. Such an apparatus and method would not only lower the manufacturing cost (by reducing the manufacturing time), but would also improve the quality of the second level package.